Aces has been American slang for "the best" at least since the first years of this century, deriving from aces, the highest cards in poker and other card games. But ace for an expert combat flier who has shot down five or more enemy planes appears to have been borrowed from the French as, "ace," during World War 1. From there ace was extended to include an expert at anything. The card name ace comes ultimately from the Greek 'as, one.